1. Field of the Disclosure
The invention relates to superconducting articles, devices and systems made therefrom, including superconductor tapes and devices.
2. Description of the Related Art
Superconductor materials have long been known and understood by the technical community. Low-temperature (low-Tc) superconductors exhibiting superconductive properties at temperatures requiring use of liquid helium (4.2 K) have been known since about 1911. However, it was not until somewhat recently that oxide-based high-temperature (high-Tc) superconductors have been discovered. Around 1986, a first high-temperature superconductor (HTS), having superconductive properties at a temperature above that of liquid nitrogen (77 K) was discovered, namely YBa2Cu3O7−x (YBCO), followed by development of additional materials over the past 15 years including Bi2Sr2Ca2Cu3O10+y (BSCCO), and others. The development of high-Tc superconductors has created the potential of economically feasible development of superconductor components incorporating such materials, due partly to the cost of operating such superconductors with liquid nitrogen rather than the comparatively more expensive cryogenic infrastructure based on liquid helium.
Of the myriad of potential applications, the industry has sought to develop use of such materials in the power industry, including applications for power generation, transmission, distribution, and storage. In this regard, it is estimated that the native resistance of copper-based commercial power components is responsible for billions of dollars per year in losses of electricity, and accordingly, the power industry stands to gain based upon utilization of high-temperature superconductors in power components such as transmission and distribution power cables, generators, transformers, and fault current interrupters. In addition, other benefits of high-temperature superconductors in the power industry include a factor of 3-10 increase of power-handling capacity, significant reduction in the size (i.e., footprint) of electric power equipment, reduced environmental impact, greater safety, and increased capacity over conventional technology. While such potential benefits of high-temperature superconductors remain quite compelling, numerous technical challenges continue to exist in the production and commercialization of high-temperature superconductors on a large scale.
Among the challenges associated with the commercialization of high-temperature superconductors, many exist around the fabrication of a superconducting tape that can be utilized for formation of various power components. A first generation of superconducting tape includes use of the above-mentioned BSCCO high-temperature superconductor. This material is generally provided in the form of discrete filaments, which are embedded in a matrix of noble metal, typically silver. Although such conductors may be made in extended lengths needed for implementation into the power industry (such as on the order of kilometers), due to materials and manufacturing costs, such tapes do not represent a commercially feasible product.
Accordingly, a great deal of interest has been generated in the so-called second-generation HTS tapes that have superior commercial viability. These tapes typically rely on a layered structure, generally including a flexible substrate that provides mechanical support, at least one buffer layer overlying the substrate, the buffer layer optionally containing multiple films, an HTS layer overlying the buffer film, and an electrical stabilizer layer overlying the superconductor layer, typically formed of at least a noble metal. However, to date, numerous engineering and manufacturing challenges remain prior to full commercialization of such second generation-tapes.
One of the more difficult challenges in creating commercially viable second generation-tapes has been the creation of a biaxially textured HTS layer. Good crystallographic biaxial texture is critical for HTS conductors to obtain high critical current (Ic) performance. The usual way to achieve the desired biaxial texture, is epitaxial growth on oriented substrate such as single crystal substrate, such as epitaxial growth of YBCO on single crystal (001) STO.
However, in order to fabricate inexpensive and flexible HTS tape for various applications, single crystal oxide substrate can not be employed, and accordingly, polycrystalline metal tape (usually <0.2 mm in thickness) have been employed as the substrate is the choice. In order to get sharp biaxial texture of HTS layer (low mosaic spread), a buffer with good biaxial texture film must be formed as the buffer upon metal tape substrate, and then the HTS layer can epitaxially grow upon the biaxially-textured buffer to obtain the desired biaxial texture.
Iijima et al., (U.S. Pat. No. 5,650,378) describes a biaxially-textured YSZ buffer layer deposited upon a polycrystalline substrate by ion beam assisted deposition (IBAD), in which an energetic, collimated ion beam is used to bombard the growing YSZ film to align the growing YSZ grains along the ion beam direction. Many researchers joined to work on this technique and showed YBCO film growing upon this textured YSZ shows excellent superconducting properties and IBAD YSZ is a robust process. However, the mechanism of forming a biaxially-textured YSZ is thought to be growth-competition based, so thick (˜1000 nm) YSZ film is required to obtained sharp texture. Accordingly, IBAD YSZ processing has been considered to be too slow for commercial production.
Wang et al. (U.S. Pat. No. 6,190,752) discloses that a biaxial texture can be formed with about 10 nm MgO by IBAD upon smooth amorphous surfaces. Accordingly, the IBAD MgO process can be much faster than IBAD YSZ process, and represents improved commercial feasibility. This biaxial texture at such a thin thickness is due to a different mechanism from the mechanism in IBAD YSZ. The quick texturing of IBAD MgO takes place during the nucleation stage during IBAD, and is not a growth-competition process as in IBAD YSZ. Wang et al. teach that in order to achieve quick texturing, a rock-salt-like material must be deposited on an amorphous substrate. However, it has been discovered that not any rock-salt-like material deposited on amorphous substrate of any material can achieve a biaxial texture, and in fact, in practice, only MgO on amorphous Si3N4 surfaces can obtain satisfactory biaxial texture upon nucleation. But due to instability of Si3N4 at a high temperature and high O2 environment required for HTS film deposition, Ic performance has been discovered to be quite poor.
Arendt et al. (U.S. Patent Application 2003/0144150) teach that an amorphous surface is not a necessity for biaxial-textured IBAD MgO, and that a good biaxial texture of IBAD MgO can also be obtained on nano-crystalline Y2O3 and with wider deposition window. Arendt et al. teach a textured rock-salt-like oxide upon crystalline oxide or oxynitride surface. As described above, not any rock-salt-like material may be deposited on crystalline substrate to obtain a suitable nucleation stage biaxial texture, in fact, in practice, only MgO on crystalline Y2O3 surfaces has been discovered to be able to obtain satisfactory biaxial texture other requirements as a buffer layer for the HTS conductor.
In an effort to develop an HTS conductor, there is a strong desire to have more choices on available biaxially-textured thin buffer beyond IBAD MgO on Y2O3 or amorphous Si3N4. The need for additional materials and processing pathways is also because IBAD MgO processing is a very delicate and difficult to control, especially compared to IBAD YSZ processing. For example, IBAD MgO processing places a high demand on smoothness (<1 nm) of Y2O3, the underlying template surface (also referred to as a nucleation seed layer). Degraded smoothness of the nucleation seed layer results in poor texture or even no texture. Another problem with processing IBAD MgO on Y2O3 is that the IBAD MgO has a very narrow optimal thickness range; as MgO thickness grows, the out-of-plane orientation changes to (111) or (110) and in-plane texture is destroyed. The parameter of atom to ion arrival ratio for IBAD MgO processing also has a very narrow window that is not wanted in large scale production of commercialization of HTS conductors.
Still further, in order to make a thin (e.g., 10 nm) IBAD MgO robust, a home-epitaxial MgO layer (˜30-80 nm) has to be grown on the IBAD MgO layer. As a drawback, Ic is very sensitive to the quality of the homo-epitaxial MgO, pacing stringent process controls on growth of this homo-epitaxial MgO. In addition, MgO has large lattice mismatch with the HTS layer (typically YBCO), so a cap layer, usually SiTiO3 has to be grown between MgO and YBCO. In addition to all these layers (at least four), generally a barrier such as Al2O3 is needed to prevent diffusion of metal elements from the substrate into the HTS layer, and to prevent oxidation of the metal substrate. Accordingly, in practice, the net structure of IBAD MgO/Y2O3 seed layer is undesirably complex, and difficult to process in a reproducible manner, especially in a large scale production setting. Accordingly, there is desire to reduce the number of layers in buffer stack to make processing less complex and reliable.
Accordingly, in view of the foregoing, there is a need in the art for improved superconductors, devices and systems incorporating such devices. In particular, there is a need for new superconducting structures having improved processability, and processes for forming commercially viable superconducting articles, such as alternative techniques of creating a biaxially textured HTS layers. In addition, further improvements in process windows for HTS conductor fabrication, additional material choices with better properties, particularly with respect to the interaction between the nucleation seed layer and IBAD textured layer, have been recognized by the present inventors as particular needs in the industry for commercialization of HTS conductors.